Photographic filter.



E. WANDERSLBR PHOTOGBAPHIG FILTER.

APPLIoA'rIon FILED onine. 190s.

Patented June 29, 1909.

mfimar UNITED STATE PATENT OFFICE.

ERNST WANDERSLEB, 0F JENA, GERMANY, ASSIGNOR TO THE FIRM OF CARL ZEISS, OF JENA, GERMANY.

PHOTOGRAPHIC FILTER.

Specification of Letters Patent.

Patented .Tune 29, 1909.

Application filed October G, 1908. Serial No, 456,481.

To all whom it may concern:

Be 1t known that I, ERNsT VANDERSLEB, a c-ltlzcn of the German Empire, and reslding vented a new and useful Photographic Filter, of which the following 1s a specification.

The present invention relates to the ycllowish filter which is applied near tho objective when making exposures upon a photoj i the objective according to the ordinary scale of distance, a final shifting has afterward to Y j be made. In focusing by means of a ground at Carl- Zeissstrasse, Jena, m the Grand i Duchy of Saxe-Weimar, Germany, have ingraphic sensitive surface, between which surface and its support a polychrome intermediate layer is situated, as for example, in the lumiere autochrome plate. The said filter presents the form of a plate or disk in which the front. and rear surfaces are plane and parallel to one another. It sometimes consists of several consecutive parts cemented or otherwise tightly united together. To form such composite yellowish filters, layers of differently colored media, for instance, have been combined or one or two colorless disks utilized as carriers or protecting covers for the filter medium proper, such medium being either fluid or solid.

Without restricting freedom in the employment of simple or variously composite filters, according to the present invention these filters are only modified with a view to rendering them available for a second purpose. As already known, in exposures upon the sensitive surfaces above mentioned, light has to penetrate the polychrome intermediate layer before it reaches the 4phot-ographic sensitive surface. The support of the sensitive surface is therefore given a position in the camera the reverse of that obtaining in ordinary photography, so that the said surface lies at the back of the support, that is to say, on the side remote from the objective. In exposures either a filter of any thickness is to be placed in front of the objective and the distance between the latter and the said support (obtaining in the case of ordinary exposures) reduced by a certain amount which is somewhat smaller than the thickness of the support; or a filter having about double the thickness of the support is to be set behind the objective. In the latter case, the reduction in distance between the objective and the su port is dispensed with, as the effect of the tiiick filter is to restore the coincidence of the image plane and the sensitive surface. When the first of these t'wo methods is followed, in focusing by shifting if y W' is only to be shifted according to the ordinary f scale or the screen used in the ordinary way. In s ite of its apparent simplicity, the second met vrodsetting the yellowish filter behind the objective-is seldom made use of, because the insertion of the filter into the interior of the camera is particularly troublesome and in addition thereto an undesirable time-wasting operation, above all when focusing by means of the screen, since the time occu ied between focusing and exposure is muc more )rolonged when the filter is to be set behind the objective than when placed in front.

By the present invention the filter placed in front of the objective is invested with the advantage hitherto only possessed by the filter set behind the objective, viz., that the ordinary modes of focusing remain in use. According to this invention a yellowish filter is made use of which has not as hitherto the effect of a homogeneous plano-parallel plate, but that of a weak dispersive lens, by virtue of one or several (external or internal) surfaces being spherical. In consequence of this filter lens acting in cooperation with the objective, the focal length of the latter is, as it were, increased little, which increase corresponds to the rearward displacement of the sensitive surface.

In the accompanying drawing a filter according to the invention is represented as applied in front of the objective of a photographic camera. It consists of a yellowish biconcave lens a', the mount Y) of which is ada ted to he slid upon the objective casing.

Tie increase in the focal length of the objective incidental to the filter lens should amount to about two-thirds of the thickness of the support, that is l mm, when this thickness is 1.5 mm. If the focal length of the objective proper be 150 mm, a negative focal length of the filter lens of 22.5 meters would just bring about this increase. The same filter lens would also suffice for objectives of 145 mm and of 155 mm focal length, without the position of the image being so much dismultiple of the focal length of the obiettive, placed relativelyv to the sensitive surface as l ure nga-in smaller than the casual dillerences 15 the casual dill'erences in the thickness ol the in the thickness ol the supports.

supports would amount to. In general, the I claim:

greater the focal length of the ohiective, the l, A vvellowish light filter' for polychrome exgreater must the focal length of the filter lens posures7 Constructed to not as a Weel( disbe chosen. persive lens and adapted to be Connected 2o The displacement of the image due to the i with the camera in front of and Coaxially to influence of the 'lilter lens is not independent 1 the objective. L3 ot' the distance of the object to be taken, but

the fluctuations in displacement, for the or- Witnesses: dlnary distances at Which photographs are l PAUL IxRGER, usually taken and which represent a high FRiTz SANDER.

ER NST VANDERSLEB. 

